Force generated by polymerization of actin filaments: an entropic role?

Actin polymerization drives protrusions at the cell surface and leads to cell motility. Using magnetic colloids, we measure how the chemical reaction of polymerization generates mechanical forces. Rapid force- distance measurement gives us access to the filaments organisation between colloids, whereas long experiments at constant forces give the force- velocity relation of growing actin filaments. A simple model based on entropic forces seems to explain our observations.